While recent advances in science and technology are helping to improve quality and add years to human life, the prevention of atherosclerosis, the underlying cause of cardiovascular disease ("CVD") has not been sufficiently addressed. Research to date suggest that cholesterol may play a role in atherosclerosis by forming atherosclerotic plaques in blood vessels, ultimately cutting off blood supply to the heart muscle or alternatively to the brain or limbs, depending on the location of the plaque in the arterial tree (1,2). Overviews have indicated that a 1% reduction in a person's total serum cholesterol yields a 2% reduction in risk of a coronary artery event (3). Statistically, a 10% decrease in average serum cholesterol (e.g. from 6.0 mmol/L to 5.3 mmol/L) may result in the prevention of 100,000 deaths in the United States annually (4).
Sterols are naturally occurring triterpenoids that perform many critical cellular functions. Phytosterols such as campesterol, stigmasterol and beta-sitosterol in plants, ergosterol in fungi and cholesterol in animals are each primary components of cellular and sub-cellular membranes in their respective cell types. The dietary source of phytosterols in humans comes from plant materials i.e. vegetables and plant oils. The estimated daily phytosterol content in the conventional western-type diet is approximately 60-80 milligrams in contrast to a vegetarian diet winch would provide about 500 milligrams per day.
Phytosterols have received a great deal of attention due to their ability to decrease serum cholesterol levels when fed to a number of mammalian species, including humans. While the precise mechanism of action remains largely unknown, the relationship between cholesterol and phytosterols is apparently due in part to the similarities between the respective chemical structures (the differences occurring in the side chains of the molecules). It is assumed that phytosterols displace cholesterol from the micellar phase and thereby reduce its absorption.
Over forty years ago, Eli Lilly marketed a sterol preparation from tall oil and later from soybean oil called Cytellin.TM. which was found to lower serum cholesterol by about 9% according to one report (5). Various subsequent researchers have explored the effects of sitosterol preparations on plasma lipid and lipoprotein concentrations (6) and the effects of sitosterol and campesterol from soybean and tall oil sources on serum cholesterols (7). A composition of phytosterols which has been found to be highly effective in lowering serum cholesterol is disclosed in PCT/CA95/00555 and comprises no more than 70% by weight beta-sitosterol, at least 10% by weight campesterol and stigmastanol. It is hypothesized in this patent application (which has already issued to patent in some countries) that there may be some form of synergy between the constituent phytosterols.
Given that phytosterols in various combinations have been proven to have wide clinical and dietary applications in lowering total and low density lipoprotein cholesterol, the key problem now facing researchers in this field is the adaptation of the phytosterol delivery system. Studies have investigated how the form (for example, crystalline, suspension, granular) in which the phytosterols are dosed impacts on their ability to lower serum cholesterol levels. Phytosterols are highly hydrophobic, do not dissolve to any significant degree in the micellar phase in the digestive tract and therefore are not capable of efficiently blocking cholesterol absorption. Oils and fats are capable to a limited but not satisfactory degree of dissolving free phytosterols. Since only solubilized phytosterols inhibit the absorption of cholesterol, this "delivery" problem must be adequately addressed.
Early research focused on grinding or milling the phytosterols in order to enhance their solubility (U.S. Pat. Nos.: 3,881,005 and 4,195,084 both to Eli Lilly). In addition, researchers have looked to the esterification of phytosterols in order to enhance their solubility in delivery systems. German Patent 2035069/Jan. 28, 1971 (analogous to U.S. Pat. No. 3,751,569) describes the addition of phytosterol fatty acid esters to cooking oil. The esterification is carried out between a free sterol and a fatty acid anhydride, with perchloric acid as the catalyst. The significant drawback to this process, along with others, is the use of non-food grade catalysts and reagents.
U.S. Pat. No. 4,588,717 to David E. Mitchell Medical Research Institute describes a vitamin supplement which comprises a fatty acid ester of a phytosterol, wherein the fatty acid forming the ester has from about 18 to 20 carbon atoms in the main carbon chain.
U.S. Pat. No. 5,270,041 to Marigen S. A. teaches the use of small amounts of sterols, their fatty acid esters and glucosides for the treatment of tumours. The method of preparation of these compositions involving the use of hazardous chemical reagents effectively precludes their use in foods or as dietary additives.
Other research has demonstrated that phytostanols, the 5 alpha saturated derivatives of phytosterols, are more effective as therapeutic agents in lowering serum cholesterol on a molecular weight basis than phytosterols (8). Similarly, in a further comparison, sitosterols infused into the GI tract resulted in a 50% reduction in serum cholesterol as opposed to an 85% reduction when sitostanols were infused (9). The advantages of stanols over sterols with respect to inhibition of cholesterol absorption from the GI tract are two-fold. Firstly, stanols are more chemically stable than their unsaturated counterparts in heat and air due to the absence of carbon-carbon bonds in the former. Secondly, stanols are more effective at lowering serum cholesterol on a molecular weight basis than their unsaturated counterparts.
U.S. Pat. No. 5,502,045 to Raision Tehtaat Oy AB (hereinafter the "Raision Patent") describes the preparation of a beta-sitostanol fatty acid ester mixture prepared by interesterifying beta-sitostanol with a fatty acid ester containing from 2 to 22 carbon atoms in the presence of an interesterification catalyst. This process renders the sitostanol appreciably more soluble in fats and oils.
South African Patent Application 967616 also to Raision Tehtaat Oy AB (hereinafter the "SA Raision Patent") describes a similar composition to that in the Raision Patent but which further contains at least 10% campestanol obtained by hydrogenation of the phytosterol mixture.
U.S. Pat. No. 5,244,887 to Straub discloses a method of making a food additive composition which comprises dissolving a stanol (sitostanol; clionastanol; 22,23-dihydrobrassicastanol; campestanol and mixtures thereof) with an edible solubilizing agent, an anti-oxidant and a carrier or dispersant.
Although the Raision Patent and the Raision SA Patent both attempt to produce a phytostanol delivery system which is stable and effective, there are significant problems with the long-term stability of these esterified products due to the ultimate oxidation of the unsaturated fatty acid moiety.
It is an object of the present invention to obviate or mitigate the above disadvantages.